One expandable endovascular stent is known from German patent No. 33 42 798, where the frame cells are formed by sets of wires extending helically through the body at opposite winding directions. The frame cells are rhomboidal, and the stent length changes substantially at the expansion, which gives rise to several disadvantages, one being that it is difficult to position the stent accurately, another that the insertion system is complicated.
U.S. Pat. No. 5,370,683 describes a stent formed from a single filament which is wrapped around a mandril in a wavy course having alternating short and long, elongated pieces of filament, whereupon the filament is arranged in a helical course with the wave troughs mutually aligned. Then the wave troughs have been interconnected to form rhomboidal frame cells having a pair of opposite short cell sides and another pair of opposite long cell sides. Among other things, this stent is distinguished in being capable of compression into a radially compressed state without the stent ends having to be pulled apart. The stent can be arranged in a catheter in a radially compressed state and be inserted and positioned at the desired site in a lumen, such as a blood vessel, whereupon the catheter can be pulled away and the stent be expanded by means of an inflatable balloon arranged inside the stent. It is a disadvantage of the stent that it has relatively poor bending flexibility, as this reduces the adaptability of the stent to the supported, flexible vessel. Nor is it an advantage that the cells of the stent are relatively open and thus more exposed to fibrous in-growth into the inner lumen of the stent.
In a stent known from EP-A 645125 a tubular stent body is also formed from a single angularly bent filament wound in a spiral shape with the apexes hooked into each other to form rhomboidal cells. Because the apexes are only hooked into each other, there is a risk of compression of the stent in the longitudinal direction if it is pushed out of the catheter. The two ends of the filament are returned through the stent body in a spiral course, but do not remove the risk of longitudinal changes in the part of the stent which is expanding outside the end of the catheter. It may, therefore, be necessary to pull the stent out of the catheter by means of a pulling device passing centrally through the stent body and restricting its compression inside the catheter. The flexibility of the stent at bending is also relatively poor, and the cells are very open.
A number of different stents of another type is also known, in which the cell material does not continue directly from one frame cell to the following one in the longitudinal direction. Instead, this type of stents is constructed from several Z-shape-bent wires joined into a tubular body by means of connecting threads or being hooked into each other, vide EP-A 622088, EP-A 480667, WO93/13825 and EP-A 556850. All these stents are of limited bending flexibility and some of them are very complicated to manufacture. The connecting threads for joining the Z-bent, resilient frame material limit the expanded stent diameter, but yield completely to axial pressure. This results in the substantial disadvantage that impacts on a cell are not transferred to the cell following in the longitudinal direction, so that the stent has discontinuous properties, can open up and will exhibit breaks at bending.
Stents constructed from wires wound about each other to form closed cells are known from DE-A 39 18 736, where the cells are elongated or .OMEGA.-shaped, and from WO94/03127, where the cells are oval in the circumferential direction.